In recent years, attention has been directed to measurement for determining the presence of a gas and its concentration thereof, and especially to measurement for determining the presence of environmental gases (e.g., CO2, NO, etc.) and its concentration thereof.
As sensors for measuring these gases with high accuracy, gas sensors using chemical reaction and optical gas sensors can be cited. In view of high measurement accuracy and small drift with time, optical gas sensors have received attention. An optical gas sensor includes a light source for emitting a wavelength that can be absorbed by molecules of a gas to be measured, and a sensor to detect a signal thereof.
The environmental gas significantly absorbs light with wavelengths around several micrometers (e.g., around a wavelength of 4.3 μm in the case of CO2), and thus, a light source for emitting light in this wavelength band and a sensor for outputting a signal in accordance with the intensity of light in this wavelength band are required. LEDs that emit light in a mid-to-far infrared range, mainly used for nondispersive infrared gas sensors (hereinafter referred to as NDIR gas sensors), have been developed.
FIG. 29A is a conceptual view illustrative of an example configuration of an NDIR gas sensor according to a first conventional example. As illustrated in FIG. 29A, the NDIR gas sensor includes a gas cell 910, a light source 920 that emits infrared rays with a wavelength corresponding to an absorption wavelength band specific to a gas, and an infrared ray sensor 930 capable of detecting an intensity of light in this wavelength band. The light source 920 and the infrared ray sensor 930 are disposed in the gas cell 910. In a NDIR gas sensor the target gas to be measured flows or is accumulated in the gas cell 910 to that the concentration of the target gas to be measured is obtained from the amount of infrared rays absorbed by a space between the light source 920 and the infrared sensor 930 in the gas cell. Thus, when the intensity of the light source of the NDIR gas sensor changes, the absolute value of the concentration of the measured gas drifts, making impossible an accurate measurement of the concentration.
FIG. 29B is a conceptual view illustrative of an example configuration of an NDIR gas sensor according to a second conventional example. As illustrated in FIG. 29B, as a generally known technique, output ratios of both a reference sensor 931 capable of detecting light in a wavelength band that is not absorbed by a detection target gas and a detection sensor 932 capable of detecting light in a wavelength band that is absorbed by the detection target gas is obtained so that output fluctuations of the light source 920 are canceled (see, for example, PTL 1).
PTL 2 discloses an NDIR gas sensor using two wavelength bands. The gas sensor of PTL 2 includes two light sources so that light with a wavelength absorbed by a detection target gas and light with a wavelength not absorbed by the detection target gas are caused to pass through a gas cell in order to measure a concentration of a gas to be detected based on output ratios of the sensors.